Microbial Mg-rich Carbonates in an Extreme Alkaline Lake (Las Eras, Central Spain)

Abstract

This paper provides strong evidence for the contribution of the phylum Firmicutes in mediating the primary precipitation of Mg-rich carbonates (hydromagnesite, dolomite, magnesite, and nesquehonite) in recent microbialites from a highly alkaline and ephemeral inland lake (Las Eras, Central Spain). The carbonate mineral precipitation occurs sequentially as the microbial mats decay. Scanning electron microscopy (SEM) provided solid proof that hydromagnesite nucleation is initiated on the exopolymeric substances (EPS) and the microbial cells associated to the microbial mat degradation areas. The progressive mineralization of the EPS and bacterial cells by hydromagnesite plate-like crystals on their surface, results in the entombment of the bacteria and formation of radiating aggregates of hydromagnesite crystals. The hydrous phases, mostly hydromagnesite, were produced at a high percentage in the first stages of the microbial degradation of organic matter. When the availability of organic substrates declines, the heterotrophs tend to reduce their number and metabolic activity, remain dormant. At this stage, the anhydrous phases, dolomite and magnesite, nucleate on bacterial nanoglobules and/or collapsed cells. Evidence for the sequential formation of the Mg-rich carbonates trough the decay of organic matter by a fermentative EPS-forming bacterium isolated from the microbialites, Desemzia incerta, is drawn through a comparative analysis of carbonate formation in both natural and experimental settings. This study will help to constrain potential mechanisms of carbonate formation in natural systems, which are of fundamental importance not only for understanding modern environments but also as a window into the geologic past of Earth and potentially Mars.

Mg-Rich Authigenic Carbonates in Coastal Facies of the Vtoroe Zasechnoe Lake (Southwest Siberia): First Assessment and Possible Mechanisms of Formation

“…Further, the specific features of the initial geometry of diatom frustules, having a high surface area and cation-binding sites [77], can contribute to the formation of nucleation zones via their service as templates for authigenic mineral nucleation. The skew-shaped form of Mg-calcite crystals in some cases may be related to the processes of biomineralization [12,78,79], although such mineral forms can also occur as result of abiotic precipitation [80]. For isolated microcrystals of Mg-Ca and Mg-carbonates that can correspond to dolomite and magnesite, it seems that carbonate nanoparticles, produced by bacteria in order to avoid cellular burial, act as the nucleation centers [81,82].…”

“…Instead, hydrated metastable phases, such as nesquehonite (MgCO 3 •3H 2 O), dypingite (Mg 5 (CO 3 ) 4 (OH) 2 •5H 2 O), and hydromagnesite (Mg 5 (CO 3 ) 4 (OH) 2 •4H 2 O) are formed under conditions of modern playas and continental water bodies [10]. At the same time, the ancient sedimentary strata are composed of magnesite, which is the most stable Mg-carbonate mineral phase [11,12]. Therefore, the question about the possible mechanism of magnesite formation is still debatable: Is it formed by the decomposition/aging of the hydrated phase of metastable compounds such as hydromagnesite [13], or is it the primary precipitation product [14]?…”

“…Another interesting aspect of this problem is related to the fact that most of the examples of Mg-carbonate formation under modern conditions are associated with algal mats [12,15,16], which in turn suggests that biomineralization is the main factor in their formation. The results of field studies and laboratory experiments demonstrated that the metabolic activity of microorganisms plays a crucial role in the precipitation of hydrous Mg-carbonates in modern aquatic environments [16][17][18].…”

“…In contrast to the vast literature related to the precipitation of Ca-carbonates and Ca-Mg-carbonates by living organisms [4,[19][20][21][22][23][24][25][26][27], the biomineralization of Mg-carbonates has been studied to a far lesser extent [12]. However, this process can provide important insights into dolomite formation and can be key to understanding the fundamental mechanisms of the evolution of the biosphere in the early stages of its development [1].…”

“…Numerous examples of modern Mg-carbonate formation under conditions of playas and saline continental lakes were reported for various arid and semi-arid regions of the world [12,14,15,[31][32][33][34][35][36][37][38][39][40][41][42][43][44][45]. The majority of such environments are evaporitic and/or alkaline [12], which is most often related to specific climatic conditions and the specific geology of territories where such phenomenon appear. For example, in the case of hydromagnesite playas of British Columbia in Canada and saline Lake Salda in Turkey, the source of Mg is related to the weathering of ultrabasic rocks [16,46,47].…”